Identification of a plasminogen binding region in streptokinase that is necessary for the creation of a functional streptokinase-plasminogen activator complex

Streptokinase is a plasminogen activator widely used to treat patients with myocardial infarction. However, streptokinase is not a protease, and must first bind and interact with plasminogen to form an enzymatic complex. By measuring the binding of recombinant streptokinase fragments to plasminogen, we have sought, first, to identify a plasminogen binding region in streptokinase and, second, to explore the relation between binding (via this region) and the generation of a functional streptokinase--plasminogen activator complex. Recombinant streptokinase bound in a saturable and specific manner to human Glu-plasminogen with a dissociation constant of 4.2 x 10(-10) M. Recombinant streptokinase fragments spanning amino acids 1-127 and 1-253 could not be shown to bind to Glu-plasminogen, whereas fragments spanning amino acids 1-352, 120-352, and 244-414 bound tightly to plasminogen and each fragment completely inhibited the binding of full-length streptokinase to plasminogen. Although these latter streptokinase fragments formed a complex with plasminogen, enzymatic assays indicated that none of them was capable of generating an active site. When the streptokinase region shared by these three fragments, spanning residues 244-352, was expressed, it also bound plasminogen and competitively inhibited the formation of a functional plasminogen activator complex by full-length streptokinase. Taken together, these data indicate that streptokinase binds to plasminogen with high affinity, that a primary binding region for plasminogen is located within amino acids 244-352, and that binding via this region is necessary for the generation of a functional plasminogen activator complex.